The present invention relates to an alkali plating bath effective for forming a zinc-iron-phosphorus alloy film through plating, a plating method using the plating bath and a method for forming a chromate film on the alloy film.
A steel plate provided thereon with a zinc-iron-phosphorus alloy film formed through plating exhibits various excellent properties. More specifically, the plate not only exhibits, for instance, excellent weldability, acceptability of a conversion treatment and compatibility with various coating layers comparable to those of a steel plate plated with a zinc-iron alloy film, but also has excellent corrosion resistance even after coating and, in particular, the steel plate is not susceptible to local corrosion and is excellent in resistance to pore-formation, due to the incorporation of phosphorus into the plated film.
For this reason, there have been proposed a variety of plating bath compositions in which the content of iron is variously changed. For instance, Japanese Un-examined Patent Publication (hereunder referred to as "J.P. KOKAI") No. Sho 59-211590 discloses a plating composition which comprises 0.0003 to 0.5% by weight of phosphorus, 0.01 to 5% by weight of iron and the balance of zinc; Japanese Examined Patent Publication (hereunder referred to as "J.P. KOKOKU") No. Sho 63-14071 discloses a plating composition which comprises 0.0003 to 0.5% by weight of phosphorus, 5 to 30% by weight of iron and the balance of zinc; and J.P. KOKOKU No. Sho 63-42717 discloses a plating composition which comprises 0.0003 to 0.5% by weight of phosphorus, 7 to 35% by weight of iron and the balance of zinc. All of the plated films disclosed in these patents are formed from acidic baths whose pH is 3 or 3.5.
However, the foregoing plating techniques using these acidic plating baths are developed for fast and continuous plating of steel plates and for forming an undercoat for painting and they, accordingly, suffer from a problem in that they are unsuitable for plating general parts which must satisfy the requirements for high glossiness and an excellent acceptability of uniform electrodeposition (in particular, the acceptability of the low current density portions). In addition, these acidic plating baths contain a large amount of strongly corrosive chlorides such as iron chloride, zinc chloride, ammonium chloride and/or potassium chloride and therefore, these techniques also suffer from a problem in that the plating installation is susceptible to corrosion due to the action of these chloride and corrosive gases generated during plating operations.